1. Field of the Invention
This invention relates to a resin-encapsulated semiconductor apparatus having a semiconductor device with a ferroelectric film, and a process for its fabrication.
2. Description of the Related Art
In recent years, non-volatile or large-capacity semiconductor memory devices having thin films of ferroelectric substances (dielectric materials having a high dielectric constant, or substances having a perovskite crystalline structure) have been proposed. Ferroelectric films have features such as self polarization and high dielectric constant characteristics. Hence, the ferroelectric films have hysteresis characteristics between polarization and electric fields of ferroelectric substances, and their utilization enables materialization of non-volatile memories. Also, the ferroelectric films have such a larger dielectric constant than silicon oxide films that memory cells can be made to have a smaller area when the ferroelectric films are used as capacitive insulation films, to enable materialization of large-capacitance highly integrated RAMs (random access memories).
The ferroelectric films are comprised of a sintered body of a metal oxide, and contain much oxygen which is rich in reactivity. When capacitors are formed by using such ferroelectric films in the capacitive insulation films, it is indispensable, in the upper and lower electrodes of the capacitive insulation films, to use a substance which is stable to oxidation reaction, as exemplified by an alloy chiefly composed of platinum.
After capacitors, interlayer insulation films and so forth have been formed, passivation films are formed on the outermost surfaces of the devices. Silicon nitride or silicon oxide is used in the interlayer insulation films and passivation films, which are usually formed by CVD (chemical vapor deposition) and hence hydrogen is often incorporated in the films.
When semiconductor apparatuses making use of such ferroelectric films are used in electronic equipment for public use, they are required to be inexpensive resin-encapsulated semiconductor apparatuses having good mass productivity. In particular, ferroelectric non-volatile memories are greatly needed for portable equipment as memories substituting flash memories, because of their properties such as low power, low voltage, and non-volatility making refresh operation unnecessary, and the resin-encapsulated semiconductor apparatuses are also desired in order to provide thin type packages.
At present, however, devices that utilize the ferroelectric films as capacitive insulation films are chiefly held by ceramic-encapsulated products, and almost no resin-encapsulated products are available. Devices with a large capacity are also not yet developed. This is because the polarization characteristics of ferroelectric films deteriorate as a result of heat treatment.
Capacitors having ferroelectric films are known to undergo deterioration of polarization characteristics upon their annealing in an atmosphere of hydrogen (Lecture Collections in '96 Ferroelectric Film Memory Technique Forum, published by K.K. Science Forum, No. 4, page 4, lines 1-12). This deterioration is presumed to be caused by the platinum of upper and lower electrodes which reacts with hydrogen to act as a reducing catalyst to reduce the ferroelectric film. In particular, in the case of large-capacity highly integrated devices, the ferroelectric films are fine in size, and hence this deterioration of the characteristics of the capacitors is forecasted to greatly affect the characteristics of the overall devices.
In the resin-encapsulating of semiconductor devices by transfer molding, encapsulant resins containing fillers (usually silica) are used. The fillers contained in encapsulant resins, however, have such hard particles that the fillers may damage the device surfaces when encapsulated. Moreover, since ferroelectric materials exhibit piezoelectricity, the characteristics of ferroelectric films may change upon application of a pressure to the ferroelectric film inside the devices when encapsulated. In the fabrication of DRAMs (dynamic random access memories), α-rays are emitted from radioactive components contained in the fillers, to cause memory soft errors in some cases. Accordingly, in order to prevent the device surfaces from being damaged by the fillers, to prevent application of pressure to the ferroelectric films and to screen α-rays being emitted from the fillers, protective films comprised of polyimide must be previously formed on the device surfaces. Such surface-protective polyimide films are formed by heat-curing polyimide precursor composition films usually at a temperature of about 350 to 450° C. When such a polyimide precursor is heat-cured, the hydrogen contained in the passivation films or interlayer insulation films may diffuse to cause a deterioration of polarization characteristics of the ferroelectric films. Thus, no resin-encapsulated products of devices in which thermoplastic resins are used as surface-protective films are known at present.